Metallic compositions which are, generally speaking, alloys which have the properties of being capable of undergoing reversible transformation from the austenitic to the martensitic state are known.
Such alloys are, for example, those disclosed in U.S. Pat. Nos. 3,012,882; 3,174,851; 3,351,463; 3,567,523; 3,753,700; and 3,759,552, Belgian Pat. No. 703,649, and in British Patent Applications, Nos. 22372/69; 55481/69; 55482/69; 55969/69; and 5373/70 (now British Pat. No. 1,315,652; 1,315,653; 1,346,046 and 1,346,047) in the name of Fulmer Research Institute, the disclosure of each of which is incorporated herein by reference.
Such alloys are also disclosed in NASA Publication SP110, "55-Nitinol-the alloy with a memory, etc." (U.S. Government Printing Office, Washington, D.C., 1972), N, Nakanishi et al, Scripta Metallurgica 5, 433-440 (Pergamon Press 1971), the disclosures of which are likewise incorporated herein by reference.
These alloys have in common the feature of undergoing a shear transformation on cooling from a high temperature (austenitic) state to a low temperature (or martensitic) state. If an article made of such an alloy is deformed when in its martensitic state it will remain so deformed. If it is heated to return it to a temperature at which it is austenitic, it will tend to return to its undeformed state. The transition from one state to the other, in each direction, takes place over a temperature range. The temperature at which martensite starts to form on cooling is designated M.sub.s while the temperature at which this process is complete is designated M.sub.f , each of these temperatures being those achieved at high, e.g., 100.degree. C per minute, rates of change of temperature of the sample, i.e., the "basic" M.sub.s, etc.. Similarly, the temperature of beginning and end of the transformation to austenite are designated A.sub.s and A.sub.f. Generally, M.sub.f is a lower temperature than A.sub.s, M.sub.s is a lower temperature than A.sub.f. M.sub.s can be equal to, lower than or higher than A.sub.s, for a given alloy composition and which also depends upon the alloy's prior thermomechanical history. The transformation from one form to the other may be followed by measuring one of a number of physical properties of the material in addition to the reversal of deformation described above, for example, its electrical resistivity, which shows an anomaly as the transformations take place. If graphs of resistivity-v-temperature or strain-v-temperature are plotted, a line joining the points M.sub.s, M.sub.f, A.sub.s, A.sub.f and back to M.sub.s forms a loop termed the hysteresis loop. For many materials M.sub.s and A.sub.s are at approximately the same temperature.
One particular useful alloy possessing heat recoverability or shape memory is the intermetallic compound TiNi, U.S. Pat. No. 3,174,851. The temperature at which deformed objects of the alloys return to their original shape depends on the alloy composition as disclosed in British Pat. No. 1,202,404 and U.S. Pat. No. 3,753,700, e.g., the recovery of original shape can be made to occur below, at, or above room temperature.
In certain commercial applications employing heat recoverable alloys, it is desirable that A.sub.s be at a higher temperature than M.sub.s, for the following reason. Many articles constructed of the alloys are provided to users in a deformed condition and thus in the martensitic state. For example, couplings for hydraulic components, as disclosed in U.S. Pat. Applications No. 852,722 (now abandoned) filed Aug. 25, 1969 (Belgian Pat. No. 755,271), and No. 51809 (now abandoned) filed July 2, 1970, are sold in a deformed (i.e., an expanded) state, the disclosures of those applications being incorporated herein by reference. The customer places the expanded coupling over the components (for example, the ends of hydraulic pipe lines) to be joined and raises the temperature of the coupling. As its temperature reaches the austenitic transformation range, the coupling returns, or attempts to return, to its original configuration, and shrinks onto the components to be joined. Because it is necessary that the coupling remain in its austenitic state during use (for example, to avoid stress relaxation during the martensitic transformation and because its mechanical properties are superior), the M.sub.s of the material is chosen to be below any which it may possibly reach in service. Thus, the recovery, during services the material will remain at all times in the austentic state. For this reason, it has to be kept in, for example, liquid nitrogen until it is used. If, however, the A.sub.s which, as used herein, means that temperature which marks the beginning of a continous sigmoidal transition, as plotted on a strain vs. temperature graph, of all the martensite capable of transforming to austenite, to the austenitic state, could be raised if only temporarily, for example, for one heating cycle, without a corresponding rise in the M.sub.s, then the expanded coupling could be maintained at a higher and more convenient temperature.
It is an object of the present invention to provide an alloy having for at least one heating cycle an A.sub.s higher than its M.sub.s, or a raised A.sub.s if the alloy already has an A.sub.s higher than its M.sub.s, and to raise the A.sub.s of an alloy, at least temporarily, relative to the M.sub.s of the alloy. Stated another way, this invention provides a means for retaining at least a useful portion of martensite at temperatures at which austenite would normally exist. Thus, the physical properties associated with martensite are retained at higher temperatures, at least until the material has been heated to a higher temperature.